South African Patent No. 78/1440 describes a tape measure where the measuring line which can be extended from and retracted into a casing is wound in the form of a close packed monolayer on a drum. As the drum is rotated, electrical signals are generated indicative of the degree of rotation of the drum, and these signals produce a digital indication of the length of line pulled from the drum.
In order for the length of line pulled from the drum to be in a constant proportion to the angular rotation of the drum it is necessary for the line to be wound on the drum in a monolayer. If the monolayer is close packed, a maximum length of line can be stored on the drum.
Patent No. 78/1440 describes a mechanism which will ensure that the line is wound back on to the drum in a monolayer, but this mechanism is rather bulky and difficult to construct.